What I meant by "breaking the game" is if they chose to destroy the dynamis currency economy solely for the sake of perpetuating the role playing aspect of it being in the past. The currency still being gil has nothing to do with the storyline and everything to do with common sense to keep things simple. You can rationalize it however you want lol. How are you going to rationalize that you will be able to buy something from the WoTG city auction house that will be tied to the Jeuno/Taz/Whitegate auction house? It's not supposed to make sense or fit with the storyline continuity. They are only going to carry the time travel theme and make explanations for it so far before they draw a line.
Actually, gil does fit the storyline, as I already explained. It did exist back then, so its perfectly viable for Gil to be the common currency. As to how the Auctionhouse can be tied to the future...
Moogles are magical beings, seriously. What other animals can carry 160 beds between 9 different locations in the blink of an eye and still call you "Kupo" without asking for any kind of payment. And hell, the fact that they dance in happiness whenever a Galka changes job and ends up naked in front of them will always remain a mystery. It honestly wouldn't surprise me if the moogles have some hidden power of super speed to bring the entire AH to every corner of the world in the blink of an eye, including the time rifts the adventurers travel to to end up in the past. Hell, I bet a single moogle could solo Dynamis - Xarcabard in the blink of an eye and bring endless destruction to the world if they so wanted. Hell, if they ever decided to start a riot by stealing our mog lockers and mog safe and cast Meteor XIV on us, I'm sure not even the combined forces of beastmen and man could stop them.
And Gatttsu, who said anything of repeatable quests? A quest that you can only do once that that takes the normal 1H or more to complete that rewards you a single dynamis currency, like most ToAU quests only reward you a single bronze coin, will
hardly be game breaking. Hell, a repeatable quest that takes 1 hour to do, once every day, with 10c as a reward, wouldn't be game breaking either, as it would take 1840 days to upgrade a relic weapon that way. The only game breaking thing about it, would be that it could lower the price of coins. But assuming it would give you 1 coin as a reward, it wouldn't change the game in any way.